The fuel burner in these boilers principally comprises biomass, bark, deinking sludge and primary sludge as well as possible auxiliary fuels such as oil and gas.
The methods of burning (firing) of, in particular, bark and sludge have developed gradually over the past few decades and bark boilers are now expected to be able to handle not only wet bark and sludge with a moisture constant of 40-65% or more, but also to be able to cope with variations of load (so-called "load swings"), all without requiring firing of auxiliary fuels. An efficient technique for burning such fuels without using auxiliary fuels has been achieved by the use of a sloping grate combined with a final burn-out grate (e.g. of the reciprocating type such as made by Kablitz in Germany) immediately downstream of this. However these known boilers with a steam capacity of at least 100 tons/hr are very large and require hanging from a support structure, the price of said structure increasing steeply for a small increase in height and width of the structure.
Many different types of boilers exist today with varying combinations of features depending on the materials to be fired and the power output required, etc. However, steam generating boilers, which may be considered as prior art for this invention, are generally constructed with a central furnace and one or more lower grates on to which fuel is fed for burning. The walls of the furnace may comprise pipes for containing the water/steam which is to be passed, in a first stage, to the water/steam separation drum at the upper part of the furnace before proceeding on to a superheating stage.
The pipes themselves are joined to one another by being attached to flat joining members of steel sheet called membranes which thus allows the formation of a continuous wall of pipes.
The boilers of the prior art are expensive to construct due to the overall number of pipes, the type of pipes involved and due to the manner in which they are attached to various structural support members. Additionally, the systems for separating and later superheating the steam are complicated, difficult to design, difficult to manufacture and fit as well as being difficult to service.
For example, the steam separation system which comprises a separation drum (e.g. a cyclone separator) normally requires several pipes each extending from the upper steam side of the drum all the way to the inlets for the superheater(s), the superheaters being themselves positioned vertically in the furnace or the back draft. The fitting of such a multiplicity of pipes is inconvenient and expensive, but has previously been found necessary if an even steam distribution along the drum is to be achieved. Such an even distribution is however important in order not to disturb the moisture-removing function of the de-mister element which otherwise would produce detrimental effects on the turbine, such as deposits which cause unbalance in the rotor.